The present invention relates to an apparatus for controlling the length of yarn used by a warp beam knitting machine to produce one rack of knitted fabric.
Warp beam knitting machines incorporate warp beam sections, each section comprising a multiplicity of yarns wound about it. These yarns, forming a yarn sheet, are unwound from the beam section to needle bars where the fabric is knitted. The knitted fabric is accumulated on take-up rolls for later use. The unwinding rate of the yarns delivered to the needle bars is directly proportional to the instantaneous angular velocity of the warp beam section and the instantaneous radii of the yarns about the beam section. Therefore as yarns are unwound from the beam section a constant unwinding rate is obtainable by continuously adjusting the angular velocity of the beam section.
The fabric produced by the warp beam knitting machines consists of rows of loops or stitches, each row being called a course. One course is produced for each revolution of the main shaft of the knitting machine. By definition, a rack of knitted fabric is the length of knitted fabric incorporating 480 courses, which therefore equals 480 revolutions of the main shaft. In addition, a runner-length is defined as the length of yarn unwound from a beam section when one rack of fabric is knitted.
Thus, in order to obtain a uniform rack of knitted fabric, it is necessary that the runner-length of the unwound yarns be held to within very close tolerances. In a typical situation where the runner-length is 60 inches, a deviation of even one inch will produce unacceptable knitted fabric due to puckering, i.e., wrinkling of the normally smooth fabric surface, or due to distortions in the pattern design of the fabric. Thus, if a diamond shaped pattern was sought in the knitted fabric, a wrinkled and distorted diamond pattern would occur. Such wrinkles, distortions, and lack of uniformity in the knitted fabric are, of course, undesirable. In addition, such errors in the runner-length result in increased tension on the yarn filaments which can cause the knitting needles to break.
Warp beam knitting machines presently relay on mechanical devices to regulate the unwinding rate of the yarn and thus the runner-length of the yarn. In particular, these devices:
1. mechanically measure the unwinding rate of the yarn; PA1 2. mechanically transfer this information by means of a chain and sprocket to a regulating spindle where the information is compared to a constant (control) speed worm wheel that monitors the main shaft's angular velocity. PA1 3. mechanically activate a pawl that allows a ratchet wheel to move one position when a difference in angular velocity between the spindle and worm wheel occurs; PA1 4. the ratchet wheel turning a micrometer thread spindle that axially moves a friction ring mounted on a cone driven by the main shaft; PA1 5. a second cone making contact with the friction ring being turned at a faster or slower rate depending upon the axial position of the friction ring; and PA1 6. the driven cone turning the warp beam section and thus adjusting the beam section's angular velocity.
Due to the various mechanical linkages in this device, there is a large time lag between measuring the beam section's unwinding rate and adjusting the friction ring to correct for any change in the unwinding rate. This inherent sluggishness of the present-day controllers therefore allows an undesirable unwinding rate of yarn to exist for a relatively long period of time and thereby produces knitted fabric with nonuniform course density. Also, since the unwinding rate adjustment is relatively coarse, the adjustment inherently overshoots or undershoots the desired unwinding rate and consequently causes the actual unwinding rate to oscillate between two values that bracket the desired unwinding rate. This relatively large oscillating variation of the actual unwinding rate causes the knitted fabric to have noticeable puckering where each change in unwinding rate occurs. Thus, a rack of knitted fabric produced by a knitting machine using present-day controllers has neither the desired course density -- i.e., uses an undesired runner-length of yarn -- nor a uniform course density.
Furthermore, present-day controllers inherently drift from the desired runner-length which necessitates that the operator periodically shut down the knitting machine in order to measure the actual runner-length and make manual adjustment to the driven cone position. Such stoppages further reduce the productivity of knitting machines.
The present invention is able to eliminate the above-mentioned undesirable characteristics of present-day controllers. In particular, the time lag between the sensed change in the yarn unwinding rate and the corresponding control of the beam section's angular velocity is greatly reduced by elminiating the mechanical linkages found in present-day controllers. By removing the pawls and ratchet wheel the present invention is able to eliminate the oscillatory movement of the friction ring which causes the over-correction and undercorrection of the yarn unwinding rate. Indeed, the stepping motor used in the present invention, to adjust the beam section's angular velocity, is able to make very small changes in the yarn unwinding rate that correspond to adjustments to the actual runner-length as small as one hundredth (1/100) of an inch. Therefore any overshoot or undershoot of the desired runner-length is negligible. Furthermore, since the invention is able to control the runner-length of yarn to any desired value set by the operator, there is no need for other means to be employed to measure the runner-length.
The present invention is therefore clearly distinguishable from the prior art. In U.S. Pat. No. 3,626,725 entitled "Runner Checker Apparatus for Warp Knitting Machines", a device is disclosed for measuring the length of yarn fed from a warp beam section of a warp beam knitting machine. This patent discloses an apparatus which is able to display the runner-length of yarn as each rack of fabric is knitted. The disclosed apparatus employs a means for counting pulses related to the yarn unwinding rate, whereby this count is terminated after 480 main shaft revolutions have occurred. This count is then displayed in terms of the runner-length of yarn fed from the beam section. Thus the disclosed apparatus automatically measures the actual runner-length but does not correct it in any manner.
The present invention allows the operator to manually select the desired runner-length by appropriately setting thumb wheel switches. Thus, the present invention does not utilize a counting technique as described in U.S. Pat. No. 3,626,725, but instead controls the runner-length of yarn by continuously monitoring the unwinding rate of the yarn and simultaneously adjusting this unwinding rate continuously to yield the desired runner-length.
The present invention is also clearly distinguishable from U.S. Pat. No. 3,543,360 entitled "Yarn Inspector", wherein a yarn defect detection device is disclosed incorporating a yarn length measuring device for purposes of displaying the defects as a function of yarn length. The present invention does not employ a yarn length measuring device, but instead continuously adjusts the unwinding rate of the yarn to yield a desired runner-length. U.S. Pat. No. 3,543,360 does not employ any type of yarn unwinding rate control device, but merely detects and displays defects in the yarn sheet per selected length increment of the yarn sheet.
Furthermore, U.S. Pat. No. 3,648,338, entitled "Automatic Tension Control Apparatus", does not anticipate the present invention. This patent discloses a device that automatically controls the packing density of filaments on a reel. It utilizes information of the filament speed and information of the desired filament speed to adjust the reel's angular velocity so as to wind the filaments with a predetermined packing density. In addition to being directed toward a different invention, that patent does not teach the use of sampling an error signal related to the difference between a desired and an actual variable and reducing this sampled error to zero by monitoring the amount of control information sent to a parameter-adjusting device. That patent also requires a knowledge of the accumulated number of reel revolutions in order to generate a desired filament speed; whereas the present invention does not require or use such information to control a beam section of a warp beam knitting machine. Furthermore, U.S. Pat. No. 3,648,338 fails to teach the use of an error display system which shows the deviation between the actual and desired variable as a function of the amount of control information sent to the parameter-adjusting device.